A known and successful system for detecting small quantities of gas in the environment is by the use of absorption spectroscopy (see U.S. Pat. No. 7,352,463 B2 to Bounaix, hereby incorporated by reference herein and included herein as Appendix A) and portable analyzers that incorporate this technique in combination with a Herriott cell are commercially available (see GAZOMAT™ INSPECTRA® natural gas leak portable analyzer). By this technique, a light beam of a selected frequency that is highly absorbed by the particular gas for which the instrument is designed is passed through a sample of the gas. The rate of absorption of the light beam is used as an indicator of the level of concentration of the gas in the sample. To increase the light beam's length of travel through the gas, the Herriot (multi-path) cell is used.
The particular gas can be methane, butane, propane, ethane, oxygen, hydrogen, nitrogen, water vapor, hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen sulfide, ammonia, carbon monoxide, carbon dioxide, nitrogen oxide, nitrogen dioxide, sulfur hexafluoride, or another gas of interest. For example, the level of concentration of methane in a gas sample can be determined by initiating a light beam at a frequency that is highly absorbed by methane and passing the beam through the gas sample. To determine whether the methane is from a natural gas or a biogas source requires further discrimination.
Discrimination of natural gas and biogas is accomplished using either gas chromatography systems or cavity ring-down spectroscopy (“CRDS”). Gas chromatography involves a long response time, is not very sensitive, and requires regular calibration. CRDS is expensive and can only be used with a leak survey car.
A need exists for an absorption spectroscopy system and method that can detect a first gas, like methane, and then immediately shift to detect two or more isotopes of that gas or detect an entirely different second gas.